The present invention relates to a method for producing metal ions and, more particularly, to an improved ion implantation method of a metal element.
A solid metal element or solid metal halide has been conventionally used as an ion source feed material for ion implantation of a metal element. In the ion implantation, a gas containing a predetermined metal element must be maintained in a plasma discharge state in an arc discharge chamber. In order to produce the gas plasma containing such a metal element, a gas containing a metal element vapor must be introduced into the arc discharge chamber. However, most of the metal elements or metal halides are solid under the normal temperature and pressure. Therefore, the solid metal element or solid metal halide has been used as the ion source feed material as described above.
The following problems are presented by a method using the solid metal element or solid metal halide as the ion source feed material:
(1) A heating oven is disposed adjacent to an ionization chamber (arc discharge chamber) and a metal vapor is produced in the heating oven. However, since the pressure of the metal vapor is controlled indirectly by controlling a temperature of the heating oven, the pressure of the metal vapor cannot be controlled properly.
(2) Since a given time interval is required to produce a gas plasma in a predetermined discharge state, an ion beam cannot be quickly produced. Furthermore, a given time interval is required to cool an oven, an arc discharge cannot be quickly stopped.
(3) Since the pressure of the metal vapor is unstable, the discharge state of the gas becomes unstable, and the ion beam becomes unstable accordingly.
(4) The discharge time interval is determined by the amount of the solid metal source introduced in the heating oven. In mass production, every time the solid metal source is consumed, a new solid metal source must be replenished. During the replenishing operation, ion implantation must be interrupted.
(5) When the metal halide is used as the solid metal source, more care must be taken since it has a high deliquescence. Further, when the interior of the ion implantation device is exposed to the ambient air after ion implantation, the metal halide which is attached to the inner wall of the device absorbs moisture in the air. The metal halide deposited on the inner wall of the device, that is, all the inner walls of the vacuum container member including the arc discharge chamber becomes viscous. Thus, the device itself is corroded by the viscous material and the vacuum state cannot be maintained in the device, resulting in serious damage.
The above problem is caused by the fact that ion source feed material such as In, Sb and GaI.sub.3 are solid state material at the normal temperature.